Field of the Invention
The present invention relates to an organic light emitting display device with enhanced color purity according to viewing angle.
Discussion of the Related Art
Organic light emitting display devices, which one form of flat panel display device, are self-emissive devices and have faster response speed, higher luminous efficacy, higher luminance, and wider viewing angle than other flat panel display devices. An organic light emitting display device includes an anode, a cathode facing the anode, and an organic emission layer (EML) disposed therebetween. Holes injected from the anode and electrons injected from the cathode are recombined in the organic EML, forming excitons, which are electron-hole pairs and the excitons return to the ground state, thus releasing energy, whereby light is emitted.
As a method of enhancing optical efficiency by effectively extracting light emitted from an organic EML, microcavity is used. In a top emission structure, microcavity is based on a principle in which light is repeatedly reflected by a reflective electrode (e.g., an anode) and a semi-transparent electrode (e.g., a cathode) spaced a set distance apart (i.e., an optical path length), and strong interference effects between these light beams occurs and thus light having a particular wavelength is amplified and light having other wavelengths excluding the particular wavelength is eliminated.
In a microcavity structure, however, an optical path at the front and an optical path at the side differ and thus a wavelength of light that causes resonance is changed. Accordingly, the optical path at the side with a greater viewing angle than that of the front is relatively long and resonance light emitted is shifted towards short wavelengths. That is, as illustrated in FIG. 1A, a maximum electroluminescent peak of a red light emitting cell is shifted towards short wavelengths by about 75 to 85 nm at a viewing angle of 60° as compared to a viewing angle of 0°, as illustrated in FIG. 1B, a maximum electroluminescent peak of a green light emitting cell is shifted towards short wavelengths by about 36 to 50 nm at a viewing angle of 60° as compared to a viewing angle of 0°, and as illustrated in FIG. 1C, a maximum electroluminescent peak of a blue light emitting cell is shifted towards short wavelengths by about 10 to 14 nm at a viewing angle of 60° as compared to a viewing angle of 0°. This is because, when a non-uniform doping region is formed in an EML, light is emitted by an undoped host. Accordingly, a color shift phenomenon according to viewing angle occurs and thus a conventional organic light emitting display device has reduced reliability in color purity.